Chat Application

Build a production-ready chat platform with direct and group messaging, room permissions, message history, attachments, notifications, and presence indicators. Define explicit models for users, rooms, memberships, messages, attachments, delivery state, and read state. Handle concurrent message delivery, reconnect behavior, offline state recovery, and safe attachment access. Include observability for realtime delivery failures, anti-abuse controls, auditability for moderation actions, and resilience when notification or media services degrade.